The present invention generally relates to mechano-electro conversion elements that convert a stress to an electric signal and more particularly to a highly miniaturized thin film strain sensor or stress sensor.
Strain sensors are used extensively for stress measurement, strain measurement, pressure measurement, or the like, in the field of automobiles, aircrafts, space vehicles, automatic control systems, medical appliances, pressure measuring devices, and the like. In these applications, there is a need of a compact and reliable strain sensor capable of measuring a stress in the range of 1 MPa-10 GPa.
Patent Reference 1
Japanese Laid-Open Patent Application 60-253750
Patent Reference 2
Japanese Laid-Open Patent Application 62-200544
Conventionally, various strain sensors are known, such as metal film strain sensor that uses a metal film for strain detection, optic strain sensor that detects strain by using photoelastic phenomenon, semiconductor strain sensor that uses semiconductor thin film for stress detection, and the like.
Among the foregoing, a metal film sensor has the problem of high cost and occupying large area. On the other hand, an optic strain sensor has high sensitivity and is thus capable of conducting high-precision measurement. However, an optic strain sensor is extremely fragile and occupies a large space, and applicability thereof to industry is limited. A semiconductor strain sensor has an advantage in that it can be manufactured with low cost as compared with the metal film strain sensor. However, with a semiconductor strain sensor, there are problems in that the operational temperature is limited or a temperature control is needed. Further, there are problems that the response is slow and needs a large area.
FIG. 1A shows an example of a metal film strain sensor according to a related art of the present invention.
Referring to FIG. 1A, a metal film 3 is attached on a specimen 1 subjected to the stress measurement via an adhesive layer 2, and the strain of the specimen 1 is calculated by measuring the resistance of the metal film 3 via terminals 3A and 3B.
With such a construction, there is a need of causing to flow an electric current through the metal film 3 in a direction parallel to the specimen 1, and thus, the strain sensor inevitably occupies a large area. Further, because a very thin metal film has to be attached, handling of the strain sensor is difficult, and there is further imposed various restrictions on the operational environment such as ambient, temperature, and the like.
FIG. 1B shows an example of a semiconductor strain sensor according to a related art of the present invention, wherein those parts corresponding to the parts explained previously are designated by the same reference numerals and the description thereof will be omitted.
Referring to FIG. 1B, a silicon chip 4 is attached to the specimen 1 via the adhesives layer 2, and the stress applied to the specimen 1 is calculated by measuring the resistance of the silicon chip 4 via terminals 4A and 4B.
On the other hand, while the semiconductor strain sensor of FIG. 1 can be manufactured with low cost and handled easily, there is a problem in that the semiconductor strain sensor occupies a large area on the specimen 1 similarly to the construction of FIG. 1A, and there is a further problem in that the operational environment such as ambient, temperature, and the like, is limited.
FIG. 1C shows a further example of the semiconductor strain sensor according to a related art of the present invention, wherein those parts corresponding to the parts explained previously are designated by the same reference numerals and the description thereof will be omitted.
In the example of FIG. 1C, a silicon oxide film 5 is deposited directly on the specimen 1 and a silicon film 6 is deposited directly on the silicon oxide film 5. Further, electrodes 6A and 6B are formed on the silicon film 6 for the purpose of resistance measurement.
Because the construction of FIG. 1C does not use an adhesives layer, and because the silicon film 6 is thin, it is possible, with the construction of FIG. 1C, to measure the resistance of the silicon film with high-precision under various conditions including high temperature environment.
On the other hand, such a construction requires vacuum process for the formation of the silicon oxide film 5 and the silicon film 6, and thus, the construction of FIG. 1C cannot be produced with low cost. Further, because of the use of the silicon film 6, there is imposed a limitation in terms of ambient similarly to the construction of the FIG. 1B, and the strain sensor of FIG. 1C cannot be used in high temperature oxidizing ambient. Further, because the resistance measurement is conducted parallel on the surface of the specimen similarly to the previous example, it is not possible with the construction of FIG. 1C to avoid the problem that that the strain sensor occupies a large area.
Further, FIG. 1D shows a construction in which there is formed a conductive diffusion region 7 in the specimen 1 and the resistance of the conductive diffusion region 7 is measured via terminals 7A and 7B.
Because no adhesive layer is used with the construction of FIG. 1D, the strain sensor of FIG. 1D can be used up to high temperatures. On the other hand, with the construction of FIG. 1D, there is imposed a limitation on the material that constitutes the specimen 1 in order to allow formation of the conductive diffusion region 7. More specifically, the specimen 1 has to be made of a semiconductor material such as Si. Further, formation of such a conductive diffusion region 7 requires processes such as ion implantation and thermal annealing process, and the cost of the strain sensor is increased inevitably. Further, even with the construction of FIG. 1D, the use of the strain sensor in the high temperature oxidizing ambient is limited similarly to the strain sensor of the FIG. 1C. Also, the problem that the strain sensor occupies large area on the specimen 1 is not avoided with the construction of FIG. 1D.